BPL is a compound useful as a material of liquid crystal polymers, heat-resistant resins, etc. Known processes for preparing BPL include a process comprising oxidatively coupling 2,6-di-t-butylphenol and reducing the resulting diphenoquinone for debutylation and a process comprising sulfonating biphenyl, followed by alkali fusion. These conventional processes have disadvantages in industrial application, such as complexity of steps involved and by-production of a large quantity of inorganic salts.
On the other hand, it is known that a dialkylbenzene is oxidized with molecular oxygen to obtain a corresponding hydroperoxide, which is then decomposed in the presence of an acid catalyst (acid decomposition) to obtain a phenol compound. For example, JP-A-48-72144 (the terms "JP-A" as used herein mean an "unexamined published Japanese patent application") discloses a process for preparing hydroquinone via p-diisopropylbenzene dihydroperoxide derived from p-diisopropylbenzene, and JP-A-61-93156 discloses a process for preparing 2,6-dihydroxynaphthalene via 2,6-diisopropylnaphthalene dihydroperoxide derived from 2,6-diisopropylnaphthalene.
JP-A-64-75440 discloses the process for preparing BPL via DIPBP, applied the above-described process, in which DIPBP is oxidized to obtain 4,4'-diisopropylbiphenyl dihydroperoxide (hereinafter abbreviated as DHP), which is then decomposed in the presence of an acid catalyst to obtain BPL.
The process of JP-A-64-75440 produces the following oxidation products including DHP.
______________________________________ Product (abbreviation) Structural Formula ______________________________________ DHP ##STR1## 4-(2-Hydroxyisopropyl)-4'-(2- hydroperoxyisopropyl)biphenyl (HHP) ##STR2## 4,4'-Bis(2-hydroxyisopropyl)- biphenyl (DCA) ##STR3## 4-(2-Hydroperoxyisopropyl)-4'- isopropylbiphenyl (MHP) ##STR4## 4-(2-Hydroxyisopropyl)-4'- isopropylbiphenyl (MCA) ##STR5## ______________________________________
Of these oxidation products, HHP and DCA, when subjected to acid decomposition as such, produce no BPL but 4-(4-isopropenylphenyl)phenol or 4,4'-diisopropenylbiphenyl as dehydration products. Therefore, acid decomposition of HHP and DCA is carried out in the presence of 1 mole or more of hydrogen peroxide per mole of the 2-hydroxyisopropyl group to obtain BPL.
However, in the above-described process, when a conversion from DIPBP to DHP+HHP+DCA is increased in order to increase a yield of BPL, a production rate of DHP is decreased while those of HHP and DCA are increased. When, in particular, the oxidation reaction is performed until the proportion of substances with their isopropyl group remaining unreacted, such as MHP, MCA, and unreacted DIPBP, is reduced to 10% or less, the proportion of DHP in the oxidation product (DHP+HHP+DCA) becomes 35% or less. As a result, the amount of hydrogen peroxide to be added for acid decomposition should be increased in proportion to the increase of the 2-hydroxyisopropyl group.
Moreover, since oxidation reaction of DIPBP involves an induction period in its initial stage, the reaction must be conducted in the presence of a radical initiator.